1. Field of the Invention
The invention relates to a grinding method especially for grinding simultaneously plural grinding portions of a workpiece by plural grinding wheels moved individually each other.
2. Description of the Related Art
It is typically known a cylindrical grinding machine having plural grinding wheels individually moved each other to grind simultaneously plural grinding portions of a workpiece. F or example, this technology is disclosed in the U.S. Pat. No. 6,409,573. By the typically known cylindrical grinding machine, as shown in FIG. 1, an individual grinding wheel T1, T2 grinds plural grinding portions K1, K2 of a workpiece W at the same time. Where these kinds of grinding are performed on a plurality of the workpiece W, a termination of a final grinding is different each other in accordance to each workpiece W. The reason of this fact is that a sharpness of cutting grain of the grinding wheel T1, T2 is different, a surface stiffness of the grinding portion K1, K2 of the workpiece is different in each workpiece W, a dimension of the grinding portion K1, K2 are different in each workpiece W, and so on. Especially, it is easily happened that a termination sequence of grindings are different according to each workpiece W, for example, the grinding portion K1 is terminated prior to the termination of the grinding portion K2 in one workpiece and the grinding portion K2 is terminated prior to the termination of the grinding portion K1 in other workpiece.
In a so-called in-process control of the typically known grinding machine where a dimension of the grinding portions of the workpiece is actually measured and the grinding is controlled based on the measured result, even if a grinding condition such as a velocity of grinding infeed is changed during the grinding by each grinding wheel, it is easily also happened that times of terminations of grindings are different according to each workpiece W. FIG. 2 shows a relationship between a grinding wheel position and a time and the position explains an end of the grinding wheel surface so that FIG. 2 shows grinding process according to a moving trace of each grinding wheel. The grinding process has plural processes such as a coarse grinding, an intermediate grinding and a fine grinding. Where the in-process controller controls to transmit the grinding process to a next grinding at the end of each grinding, it appears time difference between the grinding by one grinding wheel (a solid line) and the grinding by the other grinding wheel (a dotted line or a dashed line). Therefore, there happens either a delaying termination (the dashed line) or forward termination (the dotted line) of the other grinding wheel against the termination of said one grinding wheel to make said termination sequence different.
The workpiece, especially a long workpiece intends to be bended on the basis of a stress on the workpiece by a grinding force or a grinding resistance. Because of the difference of the termination of sequence by each grinding wheel, a deflection of the bended workpiece is not stable at each end of final grinding process. For example, in the grinding shown in FIG. 2, next explains in the view point of the grinding by the other grinding wheel. The grinding by the other grinding wheel is terminated prior to the termination by one grinding wheel (see the dashed line) in the stage that the workpiece is largely bended by the grinding resistance of one grinding wheel. However, the grinding by the other grinding wheel is terminated after the termination by one grinding wheel (see the dotted line) in the stage that the grinding resistance of one grinding wheel is released. In above typically known grinding method, because the deflection of the bended workpiece is not stable at the end of final grinding by each grinding wheel, a dispersion of a grinding accuracy of a roundness or a straightness and so on is occurred.